Entodesma navicula from Padilla Bay, collected near shore on
March Point by Jon Mayberry

(Photo by: Dave Cowles, July 2005)

Description: This clam has a deformed-appearing
shell, longer than high (at least in adults), with no hinge teeth nor chondrophore.
Johnson
and Snook say that the hinge
is covered internally by "an oblong white ossicle". The shell
has no radial ribs
but is often covered with a golden-brown or greenish periostracum
which, in this specimen, appears to be loose and baggy. If the periostracum
is worn off, the outside of the valves is chalky. The valves
tend to crack when dried. The inside of the valves
is at least slightly pearly, and is glossy and iridescent. It has
a continuous pallial
line and
pallial sinus. The valves
gape on the posterior end (photo), and
the periostracum
often extends past the shell at this point. Siphons
are orange (photo, but see this photo).
Length may exceed 10 cm.

How to Distinguish from Similar Species: Mytilimeria
nuttalli has valves with a nearly circular outline or higher than
long. Entodesma pictum has valves which do not gape, and is only
up to 2.5 cm long.

Geographical Range: Aleutian Islands,
Alaska to Baja CA

Depth Range: Low intertidal to 20
m

Habitat: Holes and crevices in rocks;
Holdfasts of kelp and giant barnacles at the lowest low tide

Biology/Natural History: Simultaneous
hermaphrodites,
though fertilization is external. Animals alternately release sperm
and eggs. Shell often grows to conform to the hole or crevice it
is in; or if not constrained, the anterior end grows narrow and short and
the posterior end is swollen but abruptly truncate
and gaping. Can attach by byssal
threads to its crevice. Predators include Pycnopodia
helianthoides. May contain a symbiotic pea crab Pinnixa
faba.

I have not often seen this clam. It was dug up by a student, Jon
Mayberry, near shore (approx. +1 tide level) at March Point in Padilla
Bay, July 2005

The gape in the shell at the posterior end can be clearly seen here.

View of the right valve. Note the loose periostracum.

The siphons are
said to be orange, though in this view in our tank they look more brownish-orange
or cream colored (compare below). Photo by Dave Cowles, July 2005

This individual has siphons
which are more orange-colored. Note also the foot projecting from
the anteroventral side (top right), which appears to be specialized for
holding onto rocks rather than the standard hatchet-shape seen in species
which dig in soft substrates. Photo by Dave Cowles, July 2012